1. Field of the Invention
The present invention relates to a color liquid crystal display device, particularly to a color liquid crystal display device utilizing birefringence which can change display color by applied voltage without using color filters.
2. Description of the Related Art
Usually, a liquid crystal display device is configured with a liquid crystal cell in which an aligned liquid crystal is filled in between two confronting sheets of glass substrates, and an electrode pattern is formed on each of the two sheets of glass substrate. The display is performed through movement of the liquid crystal molecules caused by applying voltage on the liquid crystal layer between the electrodes.
In an ordinary color liquid crystal display device which performs color display with such a liquid crystal display device, each pixel of the liquid crystal display device is utilized as an optical shutter, and one unit of color is displayed by pixels in sets of three in which each of the pixels is provided with a red (R), green (G) or blue (B) color filter.
A disadvantage in this case is that when, for instance, only red is displayed as a displaying color, only those pixels provide with the red color filter are in a display state out of the sets of three pixels, leaving the other pixels with green and blue color filters in a shielded state, which results in making the theoretical utilization efficiency of the incident light about one third.
Consequently, when it is used as a transmission-type display devices where a back light is used, there is no big disadvantage. However, when it is used as a reflection-type display device, where external light is used, since a sufficient amount of light cannot be obtained, the display device performs poorly due to the lack of brightness in the reflecting light.
As a means to solve the above described disadvantage, a birefringence-type color liquid crystal display device is proposed, in which the coloring state of a pixel itself is changed by utilizing a phenomenon in which the color can be continuously changed in accordance with applied voltages due to the birefringence effect of a liquid crystal cell, which provides a new way to effect color display without using color filters.
Particularly in recent years, as is seen, for instance, in Japanese Patent Laid-open Bulletin No. Hei 9-5072, a new method of performing more effective color display is proposed not by utilizing only the birefringence effect caused by a single liquid crystal cell but by adding a retardation film to the liquid crystal cell and utilizing a combination of the birefringence effect of the retardation film and the liquid crystal layer.
According to this method, since color display is performed by changing the spectral characteristics of a single pixel itself, that is, by changing the coloring state of the pixel, the theoretical utilization efficiency of light is three times higher than that of the color filter method which requires three pixels to exhibit one color cell, and sufficient brightness can be obtained even when color display is performed in a reflection-type display device.
Thus, a colorful color display can be obtained with a comparatively simple structure with a relatively low cost, which makes it possible to apply this device to the display portion of portable information equipment such as a game machine, a digital watch, a portable telephone and so on.
The above described birefringence-type color liquid display device can be said to be an excellent display device when it is used as a reflection-type color liquid crystal display device under the circumstance where ambient light is very bright.
However, in order to obtain sufficient visibility when ambient light is dark, for instance at night, illumination by a back light is usually required. But since a conventional birefringence-type color liquid crystal display device uses a reflector, the light coming from the back light is blocked, so that the device cannot be used as a transmission-type display device.
Certainly, in order to make it possible to use a back light, by using not a complete reflector but a transflective reflector which can transmit incident light at some transmittance, the use of the back light becomes possible.
In such a case, however, the transmittance of the transflective reflector is from 30 to 50%, resulting in a corresponding reduction in the reflectance and the brightness required for reflection-type display becomes insufficient.
The first object of the present invention is to make a birefringence-type color liquid crystal display device that is a sufficiently bright display when used as a reflection-type display device and that is satisfactorily usable as a transmission-type display device using a back light.
In a color liquid crystal display device described in Japanese Patent Laid-open Bulletin No. Hei 9-5072, the dispositional relationship between a polarizing film and liquid crystal molecules in the liquid crystal cell is taken to form an angle with which the maximum birefringence effect can be obtained, that is, the angle between the optical axis (transmittable axis or an absorption axis) and the long axis direction of the adjacent liquid crystal molecules is formed to be about 45.degree..
The .DELTA.nd value expressed by the product of the difference .DELTA.n of the birefringence of the liquid crystal layer of the liquid crystal cell and a cell gap d which is the space between a pair of substrates, and a retardation value of the retardation film, are not so different from each other and are substantially the same values, or within 200 nm if any.
However, the above described structure is not the optimal structure for a birefringence-type color liquid crystal display device and it is difficult to make the spectral characteristics at the time of displaying in each color to be the ideal spectral characteristics. From the viewpoint of color reproduction, the above described structure is considerably inferior when compared with a color liquid crystal display device using color filters.
This is because, firstly, there is a difference between the wavelength dependence characteristics of the .DELTA.nd value of the liquid crystal cell and the retardation value of the retardation film. Besides, there is a difference in the birefringence effect between the presence and absence of twist in the liquid crystal cell and the retardation film.
Then, a second object of the present invention is to provide a birefringence-type color liquid crystal display device which solves the disadvantage of such a reduction in the color reproductivity and has superior color reproductivity.